Novel metalens uses nanofins to focus all the colors of the rainbow

18 January 2018

This flat metalens is the first single lens that can focus the entire visible spectrum of light – including white light – in the same spot and at high resolution. It uses arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. Image: Jared Sisler/Harvard SEAS.

Metalenses – flat surfaces that use nanostructures to focus light – promise to revolutionize optics by replacing the bulky, curved lenses currently used in optical devices with a simple, flat surface. The one shortfall with metalenses, however, is that they have been limited in the spectrum of light they can focus well.

Now, a team of researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) has developed the first single lens that can focus the entire visible spectrum of light – including white light – in the same spot and in high resolution. In conventional lenses, this has only ever been achieved by stacking multiple lenses. The researchers report their work in a paper in Nature Nanotechnology.

Focusing the entire visible spectrum and white light – a combination of all the colors of the spectrum – is challenging because each wavelength moves through materials at a different speed. Red wavelengths, for example, will move through glass faster than blue, so the two colors will reach the same location at different times, resulting in different foci. This creates image distortions known as chromatic aberrations.

Cameras and optical instruments use multiple curved lenses of different thicknesses and made of different materials to correct these aberrations, adding to the bulk of the devices.

"Metalenses have advantages over traditional lenses," explains Federico Capasso, a professor of applied physics and senior research fellow in electrical engineering at SEAS and senior author of the paper. "Metalenses are thin, easy to fabricate and cost effective. This breakthrough extends those advantages across the whole visible range of light. This is the next big step."

The metalenses developed by Capasso and his team use arrays of titanium dioxide nanofins to equally focus wavelengths of light and eliminate chromatic aberration. Previous research demonstrated that different wavelengths of light could be focused at different distances by optimizing the shape, width, distance and height of the nanofins. In this latest design, the researchers created units of paired nanofins that control the speed of different wavelengths of light simultaneously. The paired nanofins control the refractive index on the metasurface and are tuned to induce different time delays on the light passing through different fins, ensuring that all wavelengths reach the focal spot at the same time.

"One of the biggest challenges in designing an achromatic broadband lens is making sure that the outgoing wavelengths from all the different points of the metalens arrive at the focal point at the same time," says Wei Ting Chen, a postdoctoral fellow at SEAS and first author of the paper. "By combining two nanofins into one element, we can tune the speed of light in the nanostructured material, to ensure that all wavelengths in the visible are focused in the same spot, using a single metalens. This dramatically reduces thickness and design complexity compared to composite standard achromatic lenses."

"Using our achromatic lens, we are able to perform high quality, white light imaging. This brings us one step closer to the goal of incorporating them into common optical devices such as cameras," said Alexander Zhu, co-author of the paper.

Next, the researchers aim to scale up the lens, to about 1cm in diameter, which would open a whole host of new possibilities, such as applications in virtual and augmented reality. The Harvard Office of Technology Development (OTD) has already protected the intellectual property relating to this project and is currently exploring commercialization opportunities.

This story is adapted from material from Harvard SEAS, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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